In order to make high data rate interactive services such as video conferencing and internet access available to more residential and small business customers, high speed data communication paths are required. Although fiber optic cable is the preferred transmission media for such high data rate services, it is not readily available in existing communications networks, and the expense of installing fiber optic cable is prohibitive. Current telephone wiring connections, which consist of twisted pair media, were not designed to support the high data rates required for interactive services such as video on demand or even high speed interconnects. In response, Asymmetrical Digital Subscriber Line (ADSL) technology has been developed to increase the transmission capabilities within the fixed bandwidth of existing twisted pair connections, allowing interactive services to be provided without requiring the installation of new fiber optic cable.
Discrete Multi-Toned (DMT) is a multi-carrier technique that divides the available bandwidth of a communications channel such as a twisted pair connection into a number of frequency sub-channels. These sub-channels are also referred to as frequency bins or carriers. The DMT technique has been adopted by the ANSI T1E1.4 (ADSL) committee for use in ADSL systems. In ADSL, DMT is used to generate 250 separate 4.3125 kHz sub-channels from 26 kHz to 1.1 MHz for downstream transmission to the end user, and 25 sub-channels from 26 kHz to 138 kHz for upstream transmission by the end user. Each bin is allocated a number of bits to send with each transmission. The number of bits allocated to an ADSL system are 0, and 2-15 bits.
Prior to transmitting real-time data with an ADSL system, an initialization process occurs. During a first portion of the initialization process, an activation and acknowledgment step occurs. It is during this step that a transmit activation tone is generated following power-up of the ADSL system. Transceiver training is the next step of the initialization process. During transceiver training, the equalization filters of the ADSL system are trained and system synchronization is achieved. Next, channel analysis and exchange are performed as part of the initialization processes. During the channel analysis and exchange, the Signal to Noise Ratio (SNR) of the channels is determined, and bit loading configuration of the bins and other configuration information is transferred.
Subsequent to the initialization process, real-time data transmission begins. During real-time data transmission, proposed implementations of the ANSI standard require that each carrier be transmitted with a nominal amount of power. The nominal amount of power is proposed to be a full amount of power that is approximately the same across all bins, as only a fine power gain adjustment variation occurs between carriers. However, there are disadvantages to assigning the nominal amount of transmit power to each carrier. For example, one problem is that there is unnecessary power consumption associated with assigning a nominal amount of power to a carrier that is not transmitting any data. This occurs when the requested data rate is less than the maximum data rate achievable on the line. This additional power results in additional system costs in terms of power consumption. Another issue of transmitting power on unused bins is that as a carrier's signal is attenuated over long line distances, there is a point where data cannot be transmitted with a desired certainty. When this occurs, the bit allocation capacity of the bad bin is set to zero, however, under proposed implementations of the specification, its transmit power remains allocated to the now unused bin. Therefore, there is a high cost in power even when there is not a high data rate. Another issue with the ADSL specification is that crosstalk interference occurs when signals are being transmitted at similar frequencies on adjacent lines.
Generally, over one-half of the power consumed by a typical DMT system is consumed by the line drivers. In addition to the thermal issues associated with increased power, there is an additional problem that crosstalk from adjacent phone lines can increase line noise levels as much as 40 dB. Therefore, it would be beneficial to optimize power consumption of a DMT system, and reduce cross-talk between adjacent twisted pair wires.